Date:	Tue, 27 Oct 1998 20:14:38 +0200 (IST)
From:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
X-Sender:	eliz AT is
To:	bowman <bowman AT montana DOT com>
cc:	djgpp AT delorie DOT com
Subject:	Re: Recursive make: portable technique?
In-Reply-To:	<36337F9D.12147CDB@montana.com>
Message-ID:	<Pine.SUN.3.91.981027201412.8711C-100000@is>
MIME-Version:	1.0
Reply-To:	djgpp AT delorie DOT com

On Sun, 25 Oct 1998, bowman wrote:

> I'm really getting confused here. If command.com never sees the cd
> command, in v3.77, what breaks when bash isn't present?

Nothing breaks; it's a feature, not a bug.  Here's a somewhat lengthy
explanation.

The current working directory in DOS/Windows is a global notion (on
Windows it is global to the current virtual machine, aka DOS box).  So
when Make calls `chdir' (actually, DJGPP's `system' does that when it
gets "cd foo"), the new directory is now global for all programs,
until it is explicitly changed by another `chdir'.

In contrast, on Unix, the current working directory is local to the
program that calls `chdir'.  So when the commands invoked by Make
exit, the previous working directory is automagically ``restored''.

Since there's no such magic on DOS/Windows, Make must chdir back when
the commands it invoked exit, if it wants to restore the original
directory.  The DJGPP port of Make indeed does that, but only when
Make itself is about to exit (so you wind up in the same place where
you invoked Make); that is why "make -C foo" works in recursive
sub-makes.

However, the DJGPP port does NOT restore the original directory after
running each command line in the rules.  This is done on purpose,
because most other DOS-based Make's don't support multiple commands on
a single line like the DJGPP's port does.  The only way to have some
command run on DOS in another directory without having such a feature
is like this:

     foo: bar
	  cd bardir
	  run-program-which-makes-foo

If our Make would restore the directory after each line in the rules
exits, Makefiles which use this (and there are a lot of them) will
all break.  So we don't do this.

When Bash is your shell, Bash itself restores the directory when the
command it runs exits (this is a special feature of the DJGPP port of
Bash, since on Unix Bash doesn't have to care about this).  So every
command that needs a shell to run will preserve the directory when
Bash is the shell (a line which calls `cd' will call the shell, since
make knows that `cd' is an internal shell command).

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